I like the upgrade key thing.

Maybe for evolved (now that im typing it, it doesnt sound lik a thing Kei would have) Kei could gain a body part. of another animal for some time? Liike if he were to have a bug key or something he could shedskin to remove status/ damage over time or he could gain pincers, etc. I dont know if that was what you were looking for, but there it is.